Multiple times over the past couple weeks I've switched to the virtual desktop containing my open sourcetree window and found that the column containing the staged/unstaged file list is gone. I believe it's been resized to 0 somehow and from that state there is no way to get it back, since the handle for that column is now coincident with the handle for resizing the file contents column. Restarting the app brings it back.

Screenshot showing problem

Screenshot after restarting app